The invention relates to a method for coating a substrate by the use of redundant anode sputtering of a target on a cathode, designed as a magnetron. In this case the cathode is supplied with a negative cathode potential; and, besides the cathode, two electrodes are alternately supplied with a positive potential (anode potential) or with a negative potential.
The invention also relates to a device, which is intended for coating a substrate by the use of redundant anode sputtering and which comprises a vacuum chamber, a magnetron cathode, two electrodes and a voltage source.
Vacuum coating technology for optical applications requires transparent oxide layers. According to the current state of the art, these layers are produced, as a function of the material, with direct current methods or with alternating current methods.
To this end, magnetron cathodes—planar or cylindrical—are provided with material to be deposited and are operated in a pure argon atmosphere or in a reactive atmosphere in a cathode sputtering process (sputtering process).
The major problem with a sputtering process for oxides or nitrides lies in the fact that unfortunately not only the substrates but also all other surfaces in the coating zone are coated with material having poor conductivity. This coating also takes place on the anodes, which are used in the sputtering process. This coating of the anodes with a material having poor conductivity or with a non-conductive material impedes the current flow and in extreme cases may even totally suppress the flow of current.
Outside the vacuum this coating with a material having poor conductivity may be detected by an increase in the anode voltage. This additional voltage drop causes a loss of power and leads to instability in the coating process.
When higher requirements are imposed on the layer thickness uniformity of the substrate to be coated, it turns out that the anode, which is covered with material, results in a non-uniformity of the layer thickness. The reason for this non-uniformity of the layer thickness lies in the fact that the anode is not uniformly coated by the insulating material, so that the current flows preferably to certain zones of the anodes. This non-uniform current flow over the length of the anodes is reflected in the plasma distribution of the sputtering cathode; and then the plasma concentrates on the area that continues to be the best conductive.
In addition, this current distribution is not constant in terms of time, so that the distribution of the layer thickness changes over time.
In order to ensure a stable current distribution on the anodes, a wide variety of attempts have been made—for example, the EP 0 632 142, WO 92/09718—however, in the final end without any outstanding success.
One solution to these problems is the twin magnetron system, wherein the discharge is carried out with alternating current between two identical magnetrons. Both targets are operated alternatingly as an anode and as a cathode. In the cathode phase the surface is cleaned of the back coatings from the anode phase, so that the discharge always meets with an uncoated anode. In this way the problem of the coated anode is solved for this system. However, twin magnetrons that are operated in the alternating mode are associated with not only increased complexity but also with technological drawbacks.
If a magnetron is operated as an anode, the anode voltage is higher than in the case of an anode without a magnet system. The electrons of the discharge are impeded from penetrating into the target surface by the magnet system. This impedance by the magnetic field affects the current distribution in the discharge. The result is a stationary non-homogeneity in the discharge, which is called the “cross corner effect” in the scholarly literature.
It is true that the twin magnetron has significantly improved the temporal stability, but the local layer thickness uniformity has become worse in comparison to that in a single magnetron.
This drawback can be remedied with a RAS circuit, as described in the patent U.S. Pat. No. 6,183,605 B1. RAS stands for redundant anode sputtering—that is, cathode sputtering with an additional anode.
To this end, the circuit, depicted in FIG. 1 (state of the art) is used. In this case the magnetron is connected to the central tap, and the electrodes are connected to one of the external connectors respectively of the secondary coil of a transformer; and its primary coil is fed by the medium frequency generator Vmf.
The magnetron always remains negative; and the two electrodes alternate the polarity.
Whereas a first electrode acts as the “correct” anode in the discharge (that is, accepts a positive voltage in relation to the vacuum tank), the second electrode will have, according to the law of transformers, twice the voltage of the magnetron and, thus, be extremely negative. Thus, this second electrode draws positive ions from the magnetron discharge; and said positive ions lead to the ion bombardment of the second electrode. The result is that the electrode is ion-etched.
In the next half cycle the polarity of the electrodes is reversed so that at this stage the discharge is provided with a clean anode.
The problem here is that owing to the transformer that is used, the voltage at the negative electrode is permanently defined—that is, the value of twice the burning voltage of the magnetron.
Since the ion density in a magnetron discharge is very high, the electrode to be cleaned experiences an extensive stripping that is significantly greater than the coating in the preceding half cycle.
This stripping results not only in the abrasion of the electrodes but also in the contamination of the layers to be generated with the magnetron sputtering device.
It has been proposed to make the electrodes of the same material as the target of the magnetron. However, that leads to problems in the case of targets that have low conductivity or targets that are made of brittle materials that cannot be processed. Owing to these limitations RAS technology, which has been known for a long time, could not gain acceptance.
The solution of “hiding” the anode leads to an analogous situation. The basic idea of this method, which has been practiced for a long time, is to arrange the anodes behind apertures, so that the sputtered particles can reach the anode only after multiple surges. If the opening to the cathode is adequately small, the dwell time of the anodes can be significantly increased. Of course, the non-uniformity of the layer thickness has to be accepted, because for energy-related reasons the electron currents in the quasi-neutral plasma of the sputter discharge have to be concentrated into single paths, which in turn result in a degree of ionization that varies widely and, thus, in locally different coating rates. In the case of past requirements, which were lower, this was the way to coat on an industrial scale substrates with materials having poor conductivity. The major drawback is that the aforementioned electron paths are locally unstable, so that the layer thickness distribution on the substrates varies in an unpredictable way.
The classic arrangement for anodes is disclosed in the patent U.S. Pat. No. 4,046,659. In this case the anode carrying rod is located somewhat further away from the substrate than the target next to the cathode. This position is good from an electric viewpoint, because the charge carriers have to travel only a very short distance, but the anode surface is also directly opposite the substrate so that all of the particles starting from the anode land on the substrate. In addition, a sizeable portion of the scattering vapor travels from the magnetron cathode to this anode.